The present invention concerns magnetrons, and more particularly it concerns a magnetic circuit structure for so-called inner magnet type magnetron wherein permanent magnets are built in a vacuum enclosure.
Generally, the magnetron is used for defrosting and heating foodstuffs as it is incorporated in electronic ovens and efficiently generates microwaves. Magetrons of the type wherein magnets are provided outside a vacuum enclosure have conventionally been used widely, but there has also been proposed in the art so-called inner magnet type magnetron wherein the magnets are built inside the vacuum enclosure in order to miniaturize the magnetron and decrease the material cost.
In the inner magnet type magnetron, a cylindrical anode and end plates provided on opposite ends thereof constitute a vacuum enclosure, and a pair of permanent magnets respectively attached to one end plate and the other end plate are opposed to each other with an interaction space therebetween. A magnetic circuit is established, comprising the one permanent magnet, one end plate, cylindrical anode, the other end plate, the other permanent magnet and the interaction space, for applying magnetic flux to the interaction space. However, in the inner magnet type magnetron for which miniaturization is intended, the space between the permanent magnets and the cylindrical anode is extremely narrow so that the magnetic flux between the two becomes short-circuited and a great deal of flux leaks from the pair of the permanent magnets. Although such expensive permanent magnetic material as Alnico is used, the utility efficiency of the magnetic flux in the interaction space is therefore quite low.